Optical bio-imaging tools for OCT and DOT (October 2010 – December 2012)
This project focuses on the development of two key opto-electronic components in a swept-source optical coherence tomography (SS-OCT) three-dimensional medical imaging system that operates in the 1.7 μm wavelength region: a fast tunable laser system and a photodetector. Both components are realised as a monolithic photonic integrated circuit on an indium phosphide (InP) semiconductor chip. This integration makes that the laser cavity is short and that electro-optically controlled tuning elements can be used in the laser cavity. Thus a high tuning speed can be achieved compared to non-integrated free-space SS-OCT laser systems. The integration of the components on semiconductor chips also has the potential of mass production for such optical systems.
The wavelength range of 1.6 to 1.8 μm is relevant for biomedical imaging due to its potential for achieving an increase in penetration depth compared to shorter wavelength ranges that are more commonly used in SS-OCT. This is related to the reduced the Rayleigh scattering at longer wavelengths and this wavelength region lies in between two strong water absorption peaks forming a transmission window. Semiconductor InAs/InP(100) quantum dot (QD) based gain materials were fabricated at the COBRA research institute capable of amplifying light in the 1.6 to 1.8 μm wavelength range with a wide bandwidth. These materials were used as the main active material for the tunable lasers and the photodetectors in an optical integration scheme where optical amplifiers and passive optical waveguide components are combined monoliothically.
The performance of QD waveguide photo detectors (QD-PDs) was characterized and analysed. These detectors are based on a semiconductor layer stack containing five layers of QDs that is identical to that of the QD semiconductor optical amplifiers (QD-SOAs) used previously. Therefore the QD-PDs can be integrated perfectly with QD tunable lasers. Properties such as, dark current levels, responsivity, spectral response and dynamic behaviour were measured. From this work it was concluded that the QD-PDs are most suitable and their performance met all requirements to be used in the SS-OCT system. A rate equation model was applied to understand the carrier dynamics in the QD material. The model explained well the absorption behaviour of the QDs in the 1.6 to 1.8 μm wavelength range and shows a good match to the experimental results. An equivalent electrical circuit model was applied to analyse the electrical behaviour of the detectors.
Optical amplification properties were studied of a new QD gain material based on a single high density layer (HD) InAs QDs on InAs quantum well (QW). SOA sections using HD QD material with different lengths were fabricated and were used to measure the optical gain of those HD QDs for a range of current densities and temperatures. The measurement results show a 30% higher optical gain than previous 5-layer QD material. The new HD QDs can thus be used to improve the performance of the tunable lasers and photodetectors. A rate equation model was also applied to analyse the underlying mechanisms of HD QDs.
An existing QD tunable laser design was improved; devices were fabricated and characterized. The improvement involved a new design for a low resolution MMI-tree filter in the cavity and a new laser layout to increase the optical output power. The laser was fabricated using the same five layer QD material as the first generation laser. The laser devices are characterized and calibrated using a dedicated computer-controlled system.
Main results (publications, patents)
Y. Jiao, P. J. van Veldhoven, E. Smalbrugge, M. K. Smit, S. He, and E. A. J. M. Bente, “Measurement and Analysis of Temperature-Dependent Optical Modal Gain in Single-Layer InAs/InP(100) Quantum-Dot Amplifiers in the 1.6- to 1.8-µm Wavelength Range,” IEEE Photonics Journal, vol. 4, pp. 2292-2306, 2012.
Y. Jiao, B. W. Tilma, J. Kotani, R. Nötzel, M. K. Smit, S. He, and E. A. J. M. Bente, “InAs/InP(100) quantum dot waveguide photodetectors for swept-source optical coherence tomography around 1.7 µm,” Optics Express, vol. 20, pp. 3675-3692, 2012.
B. W. Tilma, Y. Jiao, J. Kotani, B. Smalbrugge, H. P. M. M. Ambrosius, P. J. Thijs, X. J. M. Leijtens, R. Nötzel, M. K. Smit, and E. A. J. M. Bente, “Integrated Tunable Quantum-Dot Laser for Optical Coherence Tomography in the 1.7 mm Wavelength Region,” IEEE Journal of Quantum Electronics, vol. 48, pp. 87-98, 2012.
B. W. Tilma, Y. Jiao, P. J. van Veldhoven, B. Smalbrugge, H. P. M. M. Ambrosius, P. J. Thijs, X. J. M. Leijtens, R. Nötzel, M. K. Smit, and E. A. J. M. Bente, “InP-based monolithically integrated tunable wavelength filters in the 1.6–1.8μm wavelength region for tunable laser purposes,” Journal of Lightwave Technology, vol. 29, pp. 2818-2830, 2011.